This paper proposes a simple method for estimating the sound absorption coef-ficient at oblique incidence in an anechoic environment. The frequency de-pendent sound pressure (direct sound) is first measured at one microphone po-sition without any reflecting plane. In a second step a reflecting/absorbing ma-terial is inserted and the sound pressure measured again. The direct sound and the reflected sound cause interference at the microphone position. The pattern of interference depends on the absorption characteristics of the material to be tested. By analysing the interference pattern the absorption coefficient of the test object at oblique incidence can be calculated. This method can do without any complicated instruments and calculation procedures. More specifically, it yields a fast and precise estimate of the absorption ability of larger acoustic modules and linings.
A computer program using image sources was developed to aid the design of anechoic rooms. The phase shifting caused by different distances of all image and higher-order image sources and by reflection on each boundary surface are taken into consideration. The calculated sound pressure compares well with ''draw-away'' measurements. Several parameters which influence the quality of an anechoic room can be investigated by means of this simulation programme. It shows, that the absorption coefficient of the wall linings is by no means the only qualification requirement for an anechoic room. Besides the absorption, the geometry of the room, location and size of the source, the orientation and length of a prescribed measuring path and the respective test signal all affect the acoustical quality of an anechoic room for a given purpose.
Since different uses of auditoria, such as concert, opera, theatre and ballet, need a distinctive room acoustic environment, high-quality auditoria normally serve only one or, at most, two uses, e.g. concert and opera. On the other hand, the management of a cultural site usually desires a multi-purpose thea-tre, in order to react flexibly to customers demands. Acousticians are some-times tempted to design fairly cost-intensive and sometimes almost impractica-ble ''variable acoustics''. The ''Großes Haus'' of the Staatstheater in Mainz has been redeveloped from ground. This opportunity was taken and the ''house'' was given a new room acoustic design which serves four uses in one. This was based on smoothing the room response over the whole frequency range in the auditorium and the coupled stage. By an enlargement of the volume of the auditorium and the invisible installation of novel sound absorbers, which effec-tively work at low frequencies only, this goal has been achieved. The useful acoustic energy from the stage and the orchestra pit is directed onto the audi-ence by means of reflectors. Detrimental reflections are focussed away from the spectators. The feedback from spectators, musicians and conductor regard-ing the acoustics for all uses is positive.
Musicians in orchestra pits suffer from extremely high sound pressure levels and have to perform their jobs under severe difficulties concerning the hearing of the sound of their own instrument as well as that of all other members of an ensemble. Factory workers, who are exposed to weighted noise levels exceed-ing 90 dB(A), are regularly forced to wear ear plugs in order to protect them-selves. In orchestra pits the ''noise level'' often reaches 110 dB(A) and more. Highly qualified and artistically motivated musicians however, refuse to wear ear plugs of any kind since these deteriorate the necessary communication among them. Instead, the installation of novel compact low- and mid-frequency absorber modules on the walls and overhang of the pits will be shown to tackle the problem of poor music intelligibility at its roots. This concept of improving the acoustic working conditions has been realised successfully in eight orches-tra pits to the full satisfaction of the musicians and equally important their con-ductors without affecting the transmission of the sound from the pit into the auditorium.
The philosophy about the acoustics in rehearsal rooms has changed. As work places for musicians and singers, they should not be built or restored to simu-late a specific auditorium for opera or concert performances, not only because of the totally different spatial conditions but also of the completely different act-ing environments. The room acoustic environment should ensure that musi-cians are able to hear, assess and control their own playing or singing while listening and differentiating the sounds of all other members of an ensemble. An often encountered situation is the masking of the sound produced by strings due to the dominant brass and percussion instruments. It must also enable an undisturbed communication and interaction between the various instruments and the conductor. This may be achieved by a novel concept incorporating in-novative absorber and reflector modules which damp the low frequency re-sponse of the room, take care of an even distribution of the sounds of the dif-ferent instrument groups and, most importantly, improve the transmission to the conductor. A proper room acoustic design may also help to reduce high aver-age sound pressure levels, which is the inevitable result of poor music intelligi-bility within an ensemble.
Freefield rooms are normally equipped with voluminous porous or fibrous wedges the depth of which corresponds to a quarter of a wave length of the lowest frequency to be measured. When such passive absorbers are intelli-gently combined with reactive/resonant panel absorbers, anechoic linings be-come possible, which may save valuable space and provide a smooth and re-sistive surface. When measurements are to be performed in third-octave bands, down to 50 Hz, a lining thickness of 250 mm is required. For narrow-band analyses down to 100 Hz the surface facing the source must be corrugated in a specific manner increasing the total thickness to 620 mm. For aeroacoustic wind tunnels and a large number of test beds at six automobile manufacturers and several component suppliers have been equipped with the new anechoic linings with excellent results.
The Acoustics Centre AKZ occupies 2600 m ground area and a building vol-ume of 36000 m offering space for two four-wheel drive roller test beds, a tyre noise test bed, two engine test beds, a drive train test bed and a window test bed for the investigation of the attenuation and damping properties of compo-nents. All test beds are realized as semi-anechoic rooms with innovate Broad-band-Compact-Absorbers as wall linings. The first three of these test beds fulfil free-field requirements set by VW with tolerances of 1 dB for 100 Hz to 16 kHz and 2.5 dB for 40 Hz to 80 Hz that even surpass those set by ISO 3745.
Die Lärmbekämpfung an technischen Quellen und auf dem Ausbreitungsweg des Schalls zum jeweiligen Immissionsort gelingt bei hohen und mittleren Frequenzen relativ leicht z.B. mit porösen/faserigen Schallabsorbern, -dämpfern und -schirmen sowie schweren oder mehrschaligen Trennelementen. Dagegen fällt es oft schwer, die entsprechende Aufgabe bei tiefen Frequenzen (etwa unter 250 oder 125 Hz) zu lösen. In den letzten Jahren wurden Tiefen-Schlucker entwickelt, die ihre breitbandige Wirkung auf engstem Raum entfalten. Mit neuen marktgerechten Varianten lassen sich jetzt vielfältige Probleme des Schallschutzes und der akustischen Behaglichkeit auch unter besonders beengten Verhältnissen (z.B. in engen Kanälen und kleinen oder flachen Räumen) besser lösen. Mit praktischen Anwendungen in konkreten Beispielen werden innovative Möglichkeiten der Lärmminderung aufgezeigt, insbesondere für kommunikationsintensiv genutzte Räume im Schulungs- und Dienstleistungsbereich.